Oil country tubular goods such as tubing and casing used for excavation of oil wells for exploitation of crude oil or gas oil are usually connected with each other (made up) using tubular threaded joints. In the past, the depth of oil wells was 2,000-3,000 meters, but in deep wells such as recent oil fields in the sea, the depth sometimes reaches 8,000-10,000 meters or larger. The length of oil country tubular goods is typically 10 some meters, and tubing through which a fluid such as crude oil flows is surrounded by a plurality of casings. Therefore, the number of oil country tubular goods which are connected by threaded joints reaches a huge number.
Since tubular threaded joints for oil country tubular goods are subjected in their environment of use to loads in the than of tensile forces in the axial direction caused by the mass of oil country tubular goods and the joints themselves, compound pressures such as internal and external pressures, and geothermal heat, they need to maintain gas tightness without being damaged even in such severe environments.
Typical tubular threaded joints used for connecting oil country tubular goods (also referred to as special threaded joints) have a pin-box structure. A pin, which is a joint component having male (external) threads, is typically formed on the outer surface of both ends of an oil country tubular good, and a box, which is a counterpart joint component having female (internal) threads which engage with the male threads, is typically formed on the inner surface of both sides of a coupling, which is a separate member. As shown in FIG. 1, the pin has a shoulder portion (also referred to as a torque shoulder) formed on the end surface at the tip of the pin and a seal portion formed between the end surface and the male threads. Correspondingly, the box has a seal portion and a shoulder portion located in the rear of the female threads and adapted to contact the seal portion and the shoulder portion of the pin, respectively. The seal portions and the shoulder portions of the pin and the box constitute unthreaded metal contact portions of a tubular threaded joint, and the unthreaded metal contact portions and the threaded portions of the pin and the box constitute contact surfaces of a tubular threaded joint. Below-described Patent Document 1 discloses an example of such a special threaded joint.
In order to perform makeup of this tubular threaded joint, one end (the pin) of an oil country tubular good is inserted into a coupling (box), and the male threads and the female threads are tightened until the shoulder portions of the pin and the box contact each other and interfere with a suitable torque. As a result, the seal portions of the pin and the box intimately contact each other to form a metal-to-metal seal which guarantees the gas tightness of the threaded joint.
Due to various troubles occurring during the process of lowering tubing or casing into an oil well, it is sometimes necessary to loosen a threaded joint which has been made up, raise the joint from the oil well, retighten it, and again lower it into the well. API (American Petroleum Institute) requires galling resistance such that unrepairable seizing referred to as galling does not take place and gas tightness is maintained even if tightening (makeup) and loosening (breakout) are carried out 10 times on a joint for tubing and 3 times on a joint for casing.
In order to increase galling resistance and gas tightness, a viscous liquid lubricant (lubricating grease) referred to as compound grease or dope and containing heavy metal powder has been previously applied to the contact surfaces of a threaded joint each time makeup is carried out. Such compound grease is prescribed by API BUL 5A2.
With the object of increasing the retention of compound grease and improving sliding properties, it has been proposed to subject the contact surfaces of a threaded joint to various types of surface treatment to form one or more layers such as nitride treatment, various types of plating including galvanizing and dispersion plating, and phosphate chemical conversion treatment. However, as stated below, the use of compound grease may have adverse effects on the environment and humans.
Compound grease contains large amounts of powder of heavy metals such as zinc, lead, and copper. At the time of makeup of a threaded joint, the applied grease is washed off or overflows to the outer surface, and it can have an adverse effect on the environment and especially on sea life particularly due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the work environment and the work efficiency, and there is also a concern of harm to humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) aimed at preventing marine pollution in the Northeast Atlantic, strict environmental regulations are being enacted on a global scale, and in some regions, the use of compound grease is already being regulated. Accordingly, in order to avoid harmful effects on the environment and humans during the excavation of gas wells and oil wells, a demand has developed for threaded joints which can exhibit excellent galling resistance without using compound grease.
As a threaded joint which can be used to connect oil country tubular goods without application of compound grease, the present applicant proposed in Patent Document 2 a threaded joint for steel pipes having a viscous liquid or semisolid lubricating coating and in Patent Document 3 a threaded joint for steel pipes having a solid lubricating coating.    Patent Document 1: JP 5-87275 A    Patent Document 2: JP 2002-173692 A    Patent Document 3: WO 2009/072486